Molded rubber products have been used for many purposes in the pharmaceutical industry, such as for rubber stoppers, eye dropper tips and the like. In most cases, the process of making the molded rubber product is such that large numbers of small parts like stoppers are made as efficiently as possible. Capital equipment costs are high and the need to achieve high volume production is important. Energy consumption is a significant factor in production of these molded rubber products as well, and it is desirable to have a fast and effective process which uses a minimum amount of energy.
Molded rubber products developed specifically for the pharmaceutical industry initially were made from natural rubber. Natural rubber stoppers were an early choice and continued to be a preferred choice of material. However, because of the air permeability limitations of natural rubber, the industry, in time, sought other forms of elastomeric products particularly for rubber stoppers used as closures.
While butyl rubber has a superior air permeability when compared with natural rubber, various butyl rubber materials have not been problem free in their adaptation by the pharmaceutical industry. Specifically, in view of the need for relatively fast curing times, the industry has looked to sulfur as a curing agent. Because sulfur alone does not effect a rapid cure, accelerators, such as MBT, have been added to the formulations to increase the speed of cure. Unfortunately, materials like MBT are undesirable in that, under some conditions, they break down into undesirable by-products, including some which are harmful for pharmaceutical purposes. Normally, curing at higher temperatures increases the rate of cure but also increases the rate of formation of undesirable by-products. Lower temperatures are safer from this standpoint, but are also slower and less efficient. The industry has not yet developed an effective system for making molded rubber products, particularly from butyl rubber, with curing rates during the molding step of less than about 7 to 8 minutes. This time range is almost uniformly established as a standard curing time, because of the chemistries and temperature limitations. Four minute cures, which would substantially improve the efficiency of the equipment being used, are unacceptable using conventional technology. Either the cure is ineffective or the heat necessary to effect a cure at that temperature is severely deleterious.
The use of zinc oxide as a curing agent for halobutyl elastomers is common. The zinc oxide is used as a cure activator or as the sole curing agent.
The use of zinc oxide in butyl and halobutyl rubbers has been investigated to explain the crosslinking reactions. One proposed explanation for the crosslinking mechanism proposes that zinc oxide causes a cationic mechanism leading to the formation of carbon-carbon crosslinks. In a presentation given by R. Vukov and G. J. Wilson at a meeting of the ACS Rubber Division, Denver, Colorado, Oct. 23-26, 1984, the crosslinking efficiency of some halobutyl curing reactions were studied. In that presentation, identified as Vukov et al, a model was proposed and crosslinking efficiencies were measured.
In Vukov et al, m-phenylene bis-maleimide (m-PBM) was used in a crosslinking system in combination with zinc oxide. The system was used to prepare compounds on a cool 3 inch by 8 inch mill. Micro-tensile sheets were cured to full cure at 166.degree. C. Levels above 1 part per hundred parts of rubber of m-PBM used with 5 parts per 100 parts of rubber of zinc oxide were used. Full cure required at least 1.0 phr of m-PBM. In the middle range of 1 to 3 phr, m-PBM is taught as interfering with the zinc oxide crosslinking process.
In the Vukov et al paper, crosslinking density is the important property which is being evaluated. Rate of cure is not considered, nor is there any indication at all that rate of cure improvement is possible. Nothing suggests the formation of pharmaceutically useful molded rubber products such as rubber stoppers. All that can fairly be said is that perhaps 4 phr would be needed, as evidenced by the discussion concerning FIG. 7 of Vukov et al.
There does not appear to be any suggestion that there is available a molded rubber product which can be quickly made, efficiently and at relatively low temperatures and which is suitable for use in the pharmaceutical industry.